If my PCB does not require Solder Mask, what should I do?

Solder mask layer is negative.

Here is a PCB.

If you want no solder mask in your PCB, you can draw the solder mask layer like this.

If you want your PCB all covered by solder mask, you can draw the solder mask layer like this. Tip: Please apply the outline to all layers when generating the gerber, if the solder mask layer has no pattern, it may cause error information in CAM software.